As a rule, laser beams which are used in many different technical and non-technical fields do not yet have the shape and the diameter necessary for a specific application on emerging from the source, and they therefore need to be adapted in terms of shape and size. This is carried out by means of beam formers. Different solutions on a refractive and a reflective basis are known for this in the state of the art. Compared with refractive beam formers, reflective systems have the advantage that they are already intrinsically achromatized and no adaptations are necessary with respect to the wavelengths for correcting aberrations. Moreover, beam formers for high-energy lasers can only be realized with mirrors since lenses can be destroyed in the case of power inputs of, for example, 40 kW and above. In the case of lower power ratings too, impurities or air bubbles in the glass, for example, can lead to laser light being absorbed locally which, in the worst case, can lead to the lens bursting. However, several mirrors are required in the case of reflective systems if the structure is to be carried out on-axis, and individual holders also need to be provided for the additional mirrors as a rule.
A reflective beam former is described, for example, in U.S. Pat. No. 4,205,902. The systems described there are based on the structure of a Schwarzschild telescope in which an incident beam is imaged in an image plane via a convex and a concave mirror which are arranged concentrically. Because of its design, the structure is corrected to the third order for spherical aberration, coma and astigmatism. The system described in U.S. Pat. No. 4,205,902 consists of the combination of two Schwarzschild telescopes, wherein the second is constructed backwards; here, an incident parallel beam first strikes a concave surface from which it is imaged onto a convex surface, with the result that a virtual image forms behind the convex mirror. All of the mirrors are formed spherical. For the beam enlargement, an incident beam is first directed from the convex mirror of a forwards-constructed Schwarzschild telescope onto a concave mirror of this telescope and imaged in a point. The convex mirror of a backwards-constructed Schwarzschild telescope is arranged in the beam path between the concave mirror and the image point, which convex mirror guides the beam onto a concave mirror which, in turn, produces a parallel output beam with a diameter which is enlarged compared with the input beam. The mirrors therefore all have an off-axis structure, the incident beam and the emerging beam run parallel and offset with respect to each other, wherein the vectorial direction of both beams is the same. In total, therefore, four mirrors are used in this system, each two of which are arranged concentrically, i.e., they have a common centre of curvature, but the arrangement is effected in each case off-axis.
A simple system for beam expansion or beam compression is described in Japanese unexamined patent application JP 04301613 A. The system consists of two parabolically shaped mirrors, one convex and one concave. In order to expand a beam, for example, an incident beam of small diameter is guided onto the convex surface of one of the mirrors and directed by this in the means at a right angle onto the surface of the concave parabolic mirror. There, in turn, the light leaves the mirror as an expanded beam with a diameter enlarged compared with the incident beam and runs further in the same direction as the incident beam but offset with respect to this.
In EP 0 649 042 B1 a system is described in which the beam diameter is changed and the beam runs further on-axis, wherein, in addition, the ratio of the diameter of the incident to the emerging beam can be varied. For the beam enlargement, a beam striking a parabolic convex mirror is deflected and directed onto a concave, parabolic mirror, as was already stated in connection with JP 04301613 A. The surfaces of both mirrors are arranged confocally, i.e., the focal point of the concave mirror coincides with the focal point of the convex mirror. Both parabolic surfaces are formed on a monolithic metal block. As the mirror surfaces are parabolic surfaces, the ratio of the diameter of incident to emerging beam can be changed in that the area in which the incident beam strikes the parabolic surface is varied. In this way, different curvatures can be tapped. For this, a system of two flat mirrors is used, a first flat mirror directs the incident beam onto the convex surface, from there the light is directed to the concave mirror and then onto a second plane mirror which guides the emerging bundle of parallel beams further on-axis with a diameter which is enlarged compared with the input beam bundle. By displacing the two plane mirrors towards each other, different areas of the parabolic mirror surfaces can be reached, with the result that different enlargements can be set. A disadvantage is that the paths of the two plane mirrors need to be matched to each other, which involves an expensive control system.